In a "Down Flow" system, like the ones found on these tractors, the proper water level should be so that the top tank is filled about halfway up to the fill neck. This allows for the expansion due to pressurization. Since these tractors do not have an overflow tank, if they are overfilled, it will just push the excess water out onto the ground.
The reason that a radiator cap has a pressure indicator on it (ie. 16 lbs.) is because when water is pressurized it raises the boiling point. Just like when water is placed in a vacuum the boiling point is lowered.
Make sure that you are not overfilling the radiator and make sure that the radiator cap is working properly (The factory ones are junk). This should fix your problem but, If you continue to have troubles after this, you may need to adapt an overflow catch and recovery tank like modern vehicles have on them. That way if it pushes water out it can be recovered by the tractor.